1. Field of the Invention
This invention relates to zoom lenses, and more particularly to zoom lenses having general configuration of the telephoto type.
2. Description of the Prior Art
The recent trend of reducing the size of cameras is accompanied with increasing demands for small-sized zoom lenses of short total length. Even in the field of lens shutter cameras whose lenses are not interchangeable, the desire that such lenses are also of the zoom type is becoming stronger. What is more popular with the recent public than, for example, the single lens reflex camera which, though its lens being interchangeable, is bulky and incovenient to carry about, is one whose size is compact enough to be put into a pocket of the user's clothing without hurting the user when he carries it about.
To meet such a requirement, there has been proposed a zoom lens whose every lens element is made of homogeneous material or glass, and which comprises a first lens unit by positive power followed rearwardly of a second lens unit of negative power, these two units being moved axially in differential relation to effect zooming, as, for example, disclosed in Japanese Laid-Open Patent Application No. SHO 57-201213.
However, in the case of the aforesaid configuration, in order to shorten the zoom lens in the longitudinal and lateral directions, one method is to strengthen the power of each lens unit. If so, variation of aberrations with zooming is caused to intensify. To minimize the range of variation of aberrations, the total number of lens elements may be increased to suppress the production of aberrations. But this calls for a large increase in the physical length of the zoom lens. Conversely when the power is weakened to maintain the aberrations stable throughout the zooming range, an alternative problem arises that the total zooming movement of each unit must be increased in contradistinction to the minimization of the size of the entire system.
Another problem of the 2-unit zoom lens is that, as compared with the orthodox zoom lens having four lens units or the focusing unit, variator, compensator and relay lens arranged in this order from front, the degree of convergence of light rays emerging from the positive first lens unit tends to be far higher. For this reason, the focal length of the first lens unit must be made equal to or shorter than the shortest focal length of the entire system. Or otherwise it would be very difficult to realize the desired compact zoom lens. Such increase in the refractive power of the first lens unit causes the Petzval sum to take a large positive value. For the focal lengths of the entire system to have desired values, the refractive power of the rear lens unit has to be increased in the negative sense. Therefore, it becomes more difficult to achieve good stability of aberration correction throughout the entire zooming range.